Case hardening



Patented June 5, 1934 UNITED STATES CASE HARDENING Donald Aubrey Holt, Niagara Falls, N. Y., assignor to E. I. du Pont de Nemours & Company, Ina, Wilmington, DBL, a corporation of Delaware No Drawing. Application January 27, 1932,

' Serial No. 589,308

13 Claims.

This invention relates to case hardening of ferrous metals and particularly to improvements in the use of molten cyanide baths to produce a hard case on finished or partly finished articles of iron, steel, or alloy steels.

The ordinary case hardening of ferrous articles in molten baths is accomplished by immersing the object in a. molten bath containing a cyanide compound; many varied bath compositions have been proposed for this purpose. During treatment, transfer of carbon and some nitrogen to the ferrous article takes place in the bath and a case is produced; this is commonly called carburizing from the fact that predominantly carbon is transferred to the article. It has been proposed to case harden by means of a bath containing small amounts of compounds of boron, nitrogen and carbon, whereby a case containing these three elements is produced. Carburization has likewise been accomplished by heating ferrous metal articles and sprinkling them with a coating of aluminum sulfate, borax, and potassium cyanide. When this mixture is employed as a bath however, it forms a viscous mass which oxidizes rapidly, deposits metallic aluminum on the metal being treated therewith and is expensive to maintain.

Nitriding isthe production of a high nitrogen case on ferrous articles. This is done, for example, by treating alloy steels with gaseous ammonia. This is also accomplished by treating alloy steels in fused salt baths of high metal cyanide content attemperatures below 600 C.; this method, however, produces a relatively shallow or thin case and isnot applicable to plain carbon steels.

An object of this invention is to produce a tough case having a high nitrogen content on ferrous metal articles, and, specifically to produce such a case by treatment in a fused salt bath. A still further object is to provide means whereby such case may be produced on iron or plain carbon steel, as well as on alloy steels. Other objects will be hereinafter apparent.

I have discovered that the above objects may be attained by treating ferrous metal articles in a fused salt bath which contains at least 5% of cyanogen compounds; for instance metal cyanide, a. small amount of a boron compound and substantially no alkaline earth compounds.

The presence of the boron compound decreases I ing melt will give this control of the case. I have used up to 20% by weight of boron compound, calculated as B203 with excellent results; preferably I use about 1% by weight B203. I have added the boron in the form of various compounds soluble in the bath, such as sodium borate and boron oxide; preferably I use boron oxide. I do not wish to be limited however in the amounts stated nor the compounds, since larger amounts and other compounds of boron may be used for the purpose stated within the scope of my invention.

I have obtained improved nitriding action upon steels with cyanide baths containing boron operated at from 550 C. to 860 C. The best balance between nitrogen content and diffusion depth is ordinarily obtained at 790-845 C. At 860 C. the effect of the boron is still in evidence but at this temperature the surface nitrogen con tent is markedly lower than that obtained at 825 C. At 550 C. there is a noticeable nitriding action with baths containing boron but a relatively shallow case is obtained. The results indicate that temperatures less than 550 C. could likewise be used but that in such cases very. shallow'nitrogen penetration would be obtained.

I have used boron oxide as an addition agent in fused salt baths containing from 5% to substantially 100% of metallic cyanide in the absence of alkaline earth metal compounds and in each case I have thereby increased the nitriding action of the bath.

The following examples serve to illustrate my process.

- Example 1 Two cyanide baths each containing approximately 15% sodium cyanide, 65% sodium chloride, and 20% potassium chloride, were heated to 825 0.; to one bath was added approximately 1% by weight of boron oxide. These baths were aged for two hours at this temperature and a steel bar, S. A. E. 1020, (0.15-0.25% carbon) was suspended in each bath for one hour. The bars were removed, washed free from adhering bath, and four successive layers 0.004" deep were cut from the periphery of each. The eight samples of metal thus obtained were analyzed for both carbon and nitrogen. The results were as follows:

15% NaCN both 15% f Cut number Percent Percent Percent Percent carbon nitrogen carbon nitrogen 1 0. 30 0. 85 0. l4 1. 84 2 0. 28 0.37 0. 24 0. 93 3 0. 25 0. 12 0. 13 0. 26 4. 0. 24 O. 04 0. l4 0. 03

. each .was reduced to approximately 25%.

Example 2 Two cyanide baths containing about 45% sodium cyanide and about 55% sodium chloride were heated to about 815 C. and allowed to stand at that temperature until the cyanide content of To one bath was added approximately 1% by weight of boron oxide. These baths were maintained at about 815 C. and S. A. E. 1035 steel bars were suspended in each bath for thirty minutes. The bars were then cleaned and four cuts 0.003 inches deep from each were made as described in Example 1. These analyzed as follows:

24% NaCN bath 24% NaON+1% B203 Cut number Percent Percent Percent Percent carbon nitrogen carbon nitrogen Example 3 Two cyanide baths containing about 47% potassium cyanide and about 53% sodium cyanide were heated to 555 C. To one of these baths was added approximately 1% by weight of boron oxide. Strips of steel 0.01 inches thick were suspended in each bath for two hours. The strips were cleaned, broken into small pieces and analyzed for carbon and nitrogen. The results were as follows:

47% KCN+53% NaON 47% KCN+53% bath NaCN|-l% B20 Percent Percent Percent Percent carbon nitrogen carbon nitrogen Example 4 20% NaCN mm 20% Cut number Percent Percent Percent Percent carbon nitrogen carbon nitrogen 0.52 1. 04 0. 43 l. 45 0. 53 0. 37 0. 54 0. 68 0. 43 0.08 0. 48 0.07 0. 36 trace 0. 36 Trace.

Example 5 Two 25% cyanide baths were made up; one containing sodium chloride and carbonate, and the other 20% sodium borate and 55% sodium chloride and carbonate. A S. A. E. 1020 steel bar was suspended in each of these baths for on-'- hour at 025 C. These bars were then cleaned and four successive layers 0.004" deep were cut from the periphery of each. The samples thus obtained were analyzed with the following results:

Percent Percent Percent Percent carbon nitrogen carbon nitrogen By the use of my method it is possible to use a bath with a wide range of cyanide content, and obtain an increased nitriding action upon the ferrous metal treated therewith. I have obtained cases containing from 1.02.3% nitrogen in the first 0.004 inch layer and from 0.21.0% nitrogen in the second 0.004 inch layer.

My invention is satisfactory for nitriding iron and the less expensive plain carbon steels as well as alloy steels. The deeper case produced by my method lends greater strength and wearing properties to the metal so treated than can be obtained in the ammonia nitriding process in the same length of time. This is due to the fact that the temperature of ammonia nitriding is definitely limited to the temperature at which the ammonia dissociates to such extent that the desired nitriding is obtained, while my process may be operated at much higher temperatures, which cause more rapid diffusion and deeper penetration of nitrogen into the metal. The case obtained by my process is not only deeper but also is more gradual than that produced by ammonia nitriding. At times the case produced by the ammonia method is so distinct from the rest of the metal that this case tends to break ofi, while by the use of my method the nitride case contains progressively more nitrogen nearer the surface of the metal. The results obtained by my method are also much more consistent thanthose obtained by the ammonia 'gas treatment so that one can more readily predict the depth of the case by my invention than by the use of gaseous ammonia. This fact is of great value in that a specified depth of case can be obtained by some predetermined length of treatment.

My invention may be carried out at higher temperatures than is possible with other methods of nitriding with fused salt baths, making possible a more rapid penetration of nitrogen. Thus it is not necessary to make up special, low melting salt mixtures to carry out my method. Since the amount of cyanide decomposition varies directly with the concentration of cyanide, nitriding by my method, using cyanide baths with low cyanide concentration is much more economical than using a high cyanide bath as required by prior methods. The case produced by my method shows much deeper nitrogen penetration in a much shorter time than has previously been possible.

A cyanide bath, as I refer to it in the following claims, is any bath of fused salts, which contains in excess of 5% of cyanogen compounds,

'more than 5% by wei ht of an alkali metal cyanof cyanide baths.

mainder of said bath being essentially made up of alkali metal salts commonly used in cyaniding baths.

2. The process comprising treating ferrous metal articles in a cyaniding bath consisting of more than 5% by weight of an alkali metal 'cyanogen compound and boron oxide, the remainder of said bath being essentially made up of alkali metal salts commonly used in cyaniding baths.

3. The process comprising treating ferrous metal articles in a cyaniding bath consisting of more than 5% by weight of an alkali metal cyanogen compound and 1-20% of a boron compound, the remainder of said bath being essentially made up of alkali metal salts commonly used in cyaniding baths.

4. The process comprising treating ferrous metal articles in a cyaniding bath consisting of more than 5% by weight of an alkali metal cyanogen compound and about 1% of boron-o'xida' the remainder of said bath being essentially made up of alkali'metal salts commonly used in cyaniding baths.

5. The process comprising treating ferrousmetal articles with a molten bath consisting of more than 5% by weight of sodium cyanide, a boron compound and the remainder of said bath being made up of alkali metal salts commonly used in the preparation of cyanide baths.

6. The process comprising treating ferrous metal articles with a molten bath consisting of more than 5% by weight of sodium cyanide, about 1% byweight of boron oxide and the remainder of said bath being made up or alkali metal salts commonly used in the preparation 4,962,091 ogen compound and a boron compound, the re- '7. The process comprising treating ferrous metal articles in a molten bath consisting 01' 5% to 99% by weight of an alkali metal cyanide, 20% .to 1% by weight of a boron compound and to 0% by weight of alkali metal salts commonly used as diluents in cyanide baths.

8. The process comprising treating steel articles in a molten bath consisting of 1045% by weight of sodium cyanide, approximately 1% by weight of boron oxide and'8954% by weight of alkali metal salts commonly used in cyanide baths and maintained at 780 -84? 0.

9. A composition of matter consisting of more than 5% by weight of an alkali metal cyanogen compound and a compound 01' boron, and alkali metal salts commonly used in cyanide baths.

10. A composition of matter consisting of more than 5% by weight of alkali metal cyanide, from 1-20% by weight of a boron compound and alkali metal salts commonly used in cyanide baths.

11. A composition of matter consisting-oi more than 5% by weight of sodium cyanide, about 1% by weight of boron oxide, and alkali metal salts commonly used in cyanide baths.

12. A composition of matter comprising a mix- 100 ture consisting of 5% to 99% by weight of alkali metal cyanide and 20% to 1% by weight of a boron compound, and 75% to 0% by weight of alkali metal salts commonly used as diluents in cyanide baths.

13. A composition of matter comprising a mixture consisting of 10%-45% by weight of sodium cyanide and about 1% by weight of boron oxide, and 89-54% by weight of alkali metal salts commonly used in cyanide baths. 110

DONALD AUBREY HOLT. 

